In the papermaking industry, it is required to carry out the papermaking in a closed system and achieve the recycling of waste paper and broke which contain calcium carbonate as well as to improve the quality of paper product. For satisfying these requirements, papermaking in the neutral pH range has been more commonly conducted with a less amount of crude alum used. With a reduced amount of crude alum and at a higher pH, however, the sizing effect of the conventional anionic rosin sizing agent is significantly degraded, especially in the presence of calcium carbonate. In order to compensate for the degradation of the sizing effect, a large amount of crude alum must be added, which may deteriorate the quality of the product and the problems involved in the operation and cost of papermaking remain unsolved.
Alkylketene dimers and alkenylsuccinic anhydrides are known as sizing agents useful in the neutral pH range. However, they are inferior to rosin sizing agents in sizing performance for high yield pulps such as mechanical pulp. Furthermore, the sizing effect of alkylketene dimers for paper just off-the-machine is not on a satisfactory level and picking-up control cannot be smoothly carried out in the size press, while alkenyl-succinic anhydrides cause an operational problem: contamination of the press roll and the dryer canvas.
Under the above-mentioned circumstances, rosin sizing agents which exhibit improved stability and excellent sizing effect in the neutral pH range are strongly needed.
Cationic rosin emulsion sizing agents have recently drawn attention as such sizing agents which may satisfy the requirements. It has been shown that they fix onto the pulp fibers by themselves and accordingly satisfactory sizing can be attained with a reduced amount of crude alum. It has also been shown that they exhibit an excellent sizing effect in the neutral pH range (TAPPI Papermakers Conference 1988 pp. 181-188). However, commercial scale production of a cationic rosin emulsion involves more difficulties in comparison with that of conventional anionic rosin emulsion. Furthermore, they are still to be improved in storage stability and performance of the product.
Methods for preparing a cationic rosin emulsion have been proposed, which are generally classified in two types. One method comprises emulsifying a water-insoluble rosin material by using some cationic emulsifying agent(s) and the other comprises mixing an anionic rosin emulsion with some cationic compound(s) to convert the emulsion into a cationic emulsion.
The former method is typically described in Japanese Patent Publication No. 58-34509 (corresponding to U.S. Pat. No. 3,966,654) and Japanese Laid open Patent Publication No. 63-120198 (corresponding to U.S. Pat. No. 4,943,608). Japanese Patent Publication No. 58-34509 discloses a method in which a fortified rosin is emulsified and dispersed by ahomogenizer or the like with the aid of a water-soluble cationic resin dispersant such as polyaminopolyamide-epichlorohydrin resin, alkylenepolyamine-epichlorohydrin resin and poly(diallyamine)-epichlorohydrin resin. However, since these cationic resins do not have sufficient ability to emulsify and disperse rosin materials, the storage stability of the product is not,satisfactory. Furthermore, since a significantly large amount of the cationic resin must be used in the emulsion, the cost of the product becomes high. High resin content also causes foaming in the papermaking system. Japanese Laid-open Patent Publication No. 63-120198 discloses a method for preparing a paper sizing composition by using a quaternary copolymer of a specific hydrophobic monomer and a specific cationic monomer for dispersing a fortified rosin (the hydrophobic monomer is (meth)acrylalkylester and/or a styrene compound and the cationic monomer is alkyl- aminoalkylester of (meth)acrylic acid or alkylaminoalkylamide of (meth)acrylic acid). However, this method involves problems in that, for instance, a large amount of the dispersant is needed in order to provide the product with good storage stability, which makes the emulsion inferior in foaming property and sizing performance.
The latter method is typically described in GB Patent Nos. 2141751A and 2159183A in which the use of a "one shot" sizing composition is disclosed. According to this method, a cationic emulsion is prepared by mixing a conventional anionic rosin emulsion with an organic polymer electrolyte, preferably polydi(lower-alkyl)diallyl ammonium chloride, or an inorganic cationic polymer, preferably polyaluminum chloride, by means of a high-speed mixer. The thus prepared emulsion is essentially a very unstable disperse system comprising aggregated particles of wide-ranging particle size as is clear from the statement: "The mixture of rosin emulsion and the cationic compound forms aggregates gates at first and on further mixing the mixture changes into a disperse system which is stable for a few weeks. It can be redispersed by mixing even when some separation-is observed."
As pointed out above, conventional cationic rosin emulsion sizing agents are inferior in storage stability and sizing performance especially in the neutral pH range. Improvements have been desired.
Thus the object of the present invention is to provide a paper sizing composition which has improved storage stability and exhibits an excellent sizing performance in neutral papermaking and that can be easily prepared on a commercial scale and also a sizing method using said paper sizing composition.